The ability of renal cortical tissue of most newborn species to transport organic ions is immature. Investigations in this laboratory are designed to determine factors that influence development of this system. Factors such as litter size, time of weaning and nutritional status are under investigation. Stimuli in the environment which might serve as "triggers" of this system have been identified and the mechanism of their stimulatory effect is under study. Previous studies have demonstrated that substrates of the transport system will stimulate development. Current studies using this tool of substrate stimulation are designed to identify the proteins involved in the transport processes. These might be transport proteins or those associated with the energetics of transport.